If you're one of the 20 million or so Americans with a flexible spending account for health care, get ready for some changes.

Starting Jan. 1, you'll no longer be able to set aside pretax dollars in that account to use for medicines bought without a doctor's prescription.

That change -- and another coming in 2013 that will limit the amount of money that can be saved pretax in the accounts to $2,500 per year -- is intended to help pay for the new health overhaul law. That's because every dollar a worker puts into a health FSA is a dollar that worker doesn't pay taxes on. So limiting how much the worker can save, or what the money can be used for, increases the workers' tax bill.

How Flexible Spending Accounts Work

About 1 in 4 workers has access to a health FSA plan, which is mostly offered by larger employers. These plans have been growing in popularity in recent years, particularly as the cost of health care has risen.

The idea is fairly simple: Workers have a portion of each paycheck withheld, before taxes, and deposited into the FSA. Then they draw that money down during the year to pay for whatever health expenses their health insurance doesn't cover.

"So, for example, some very common items are things like eyeglasses, kids' orthodontia, those types of expenses that aren't necessarily covered by the medical benefit plans, but certainly are a medical expense that folks often incur," says Kelly Traw of the benefits consulting firm Mercer.

But there's a catch. Whatever doesn't get used by the end of the year is forfeited. So workers must have a pretty good idea at the beginning of the year how much they will spend out of pocket and not overestimate.

Loophole

That's where the limit on over-the-counter medications comes in. It turns out there's a loophole. While it appears that you won't be able to use your FSA to buy aspirin or Tylenol -- or drugs that you used to need a prescription for but don't anymore, like the popular allergy drug Claritin or the heartburn remedy Prilosec -- that's not quite the case.

What the law actually says is that you need a prescription for it to be reimbursable from your FSA. But there's nothing to stop a doctor from writing a prescription even for an over-the-counter drug.

"So if it was an item that you can obtain a prescription for, and you get that prescription, then you can indeed use a health FSA to reimburse that," says Traw.

Pushback From Employers And Manufacturers

A lot of people who opposed the change think that loophole may lead to a lot of extra trips to the doctor.

"It makes no sense -- the whole notion of overburdening doctors to write prescriptions for aspirin or Claritin or something, just again is so wasteful," said Dennis Triplett, chairman of the Employers Council on Flexible Compensation, a coalition that tried to stop the changes to the accounts.

Triplett, whose day job is at a bank that administers FSAs and other tax-preferred accounts, says Congress went after the wrong population in paring back the FSA benefits. "Who are you really impacting?" he said. "You're impacting the people that have chronic diseases. You're impacting the young families who want to save for braces for their kids. This is not a benefit that's designed to help Donald Trump. This is for working-class folks."

Also lining up in opposition to the changes are the people who make and sell over-the-counter drugs.

David Spangler of the Consumer Healthcare Products Association, a trade group for manufacturers of nonprescription medications, said those products "are the first line of defense in a number of instances for injuries or illnesses, and ought to be reimbursable just as prescription drugs are."

Meanwhile, the National Association of Chain Drug Stores is asking for a two-year delay in the new restrictions. It's worried about problems with computer systems being able to distinguish between drugs sold with and without prescriptions.

"Maintaining access to products that keep people healthy and that make people feel better, those are very wise short-term investments because often they can prevent higher-cost, long-term alternatives," said Chris Krese, a senior vice president with the group.

And, of course, Congress is getting into the act. At least three separate bills have been introduced that would repeal the changes to the flexible spending account provisions.

But if you're one of those workers who have to decide in the next few weeks how much to put in their account for next year, you'd probably be wise not to plan for the cost of those over-the-counter medications. Just to be safe.